Re: [PATCH v2 1/3] media: v4l2-ctrls: Add average qp control

[Andrzej Pietrasiewicz <andrzej.p@collabora.com>]

Hi Ming,

W dniu 30.04.2024 o 04:20, ming qian pisze:
> 
> Hi Andrzej,
> 
>> Hi Ming Qian,
>>
>> W dniu 25.04.2024 o 08:50, Ming Qian pisze:
>>> Add a control V4L2_CID_MPEG_VIDEO_AVERAGE_QP to report the average qp
>>> value of current encoded frame. the value applies to the last dequeued
>>> capture buffer.
>>>
>>> Signed-off-by: Ming Qian <ming.qian@nxp.com>
>>> ---
>>> v2
>>>   - improve document description according Hans's comments
>>>   - drop volatile flag
>>>
>>>   Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst | 5 +++++
>>>   drivers/media/v4l2-core/v4l2-ctrls-defs.c                 | 5 +++++
>>>   include/uapi/linux/v4l2-controls.h                        | 2 ++
>>>   3 files changed, 12 insertions(+)
>>>
>>> diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst 
>>> b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>>> index 2a165ae063fb..7d82ab14b8ba 100644
>>> --- a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>>> +++ b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>>> @@ -1653,6 +1653,11 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
>>>       Quantization parameter for a P frame for FWHT. Valid range: from 1
>>>       to 31.
>>> +``V4L2_CID_MPEG_VIDEO_AVERAGE_QP (integer)``
>>> +    This read-only control returns the average QP value of the currently
>>> +    encoded frame. The value applies to the last dequeued capture buffer
>>> +    (VIDIOC_DQBUF). Applicable to encoders.
>>> +
>>
>> What is the intended range of the values? And why? Is your intention to make it
>> a hardware-agnostic control or a hardware-specific control?
>>
>> Regrds,
>>
>> Andrzej
>>
> 
> The value range depends on the format,
> For H264, it's [V4L2_CID_MPEG_VIDEO_H264_MIN_QP, V4L2_CID_MPEG_VIDEO_H264_MIN_QP],
> usually this is [0, 51].
> For HEVC, it's [V4L2_CID_MPEG_VIDEO_HEVC_MIN_QP, V4L2_CID_MPEG_VIDEO_HEVC_MAX_QP],
> usually it's from 0 to 51 for 8 bit and from 0 to 63 for 10 bit.
> For H263 and MPEG4, it may be from 1 to 31.
> For VPX, it may be from 1 to 128.
> 
> I think the driver that supports this ctrl can set an appropriate value
> range based on the format it supports.
> 
> Users also need to read the value of this ctrl according to the format
> they set.

What happens if the user does not set the format or until they set it?

This looks like a contextual control, where the context is a cross
product between setting or not setting the format and which particular
format is actually set.

I don't want to voice strong opinions about whether this should or
should not be accepted, but this kind of behavior should definitely
be documented so that driver writers know how to implement this control.
Right now reading the documentation in the *.rst file as a driver writer
I would be puzzled. So, from a driver writer's perspective I'd expect
these additions to the doc:

- the range of returned values
- the expected behavior when format has not been set
- the expected behavior when no frames have been processed yet (so there's no QP
to be read back from the hardware)

Regards,

Andrzej

> 
> Best regards,
> Ming
> 
>>>   .. raw:: latex
>>>       \normalsize
>>> diff --git a/drivers/media/v4l2-core/v4l2-ctrls-defs.c 
>>> b/drivers/media/v4l2-core/v4l2-ctrls-defs.c
>>> index 8696eb1cdd61..1ea52011247a 100644
>>> --- a/drivers/media/v4l2-core/v4l2-ctrls-defs.c
>>> +++ b/drivers/media/v4l2-core/v4l2-ctrls-defs.c
>>> @@ -970,6 +970,7 @@ const char *v4l2_ctrl_get_name(u32 id)
>>>       case V4L2_CID_MPEG_VIDEO_LTR_COUNT:            return "LTR Count";
>>>       case V4L2_CID_MPEG_VIDEO_FRAME_LTR_INDEX:        return "Frame LTR Index";
>>>       case V4L2_CID_MPEG_VIDEO_USE_LTR_FRAMES:        return "Use LTR Frames";
>>> +    case V4L2_CID_MPEG_VIDEO_AVERAGE_QP:            return "Average QP Value";
>>>       case V4L2_CID_FWHT_I_FRAME_QP:                return "FWHT I-Frame QP 
>>> Value";
>>>       case V4L2_CID_FWHT_P_FRAME_QP:                return "FWHT P-Frame QP 
>>> Value";
>>> @@ -1507,6 +1508,10 @@ void v4l2_ctrl_fill(u32 id, const char **name, enum 
>>> v4l2_ctrl_type *type,
>>>           *max = 0xffffffffffffLL;
>>>           *step = 1;
>>>           break;
>>> +    case V4L2_CID_MPEG_VIDEO_AVERAGE_QP:
>>> +        *type = V4L2_CTRL_TYPE_INTEGER;
>>> +        *flags |= V4L2_CTRL_FLAG_READ_ONLY;
>>> +        break;
>>>       case V4L2_CID_PIXEL_RATE:
>>>           *type = V4L2_CTRL_TYPE_INTEGER64;
>>>           *flags |= V4L2_CTRL_FLAG_READ_ONLY;
>>> diff --git a/include/uapi/linux/v4l2-controls.h 
>>> b/include/uapi/linux/v4l2-controls.h
>>> index 99c3f5e99da7..974fd254e573 100644
>>> --- a/include/uapi/linux/v4l2-controls.h
>>> +++ b/include/uapi/linux/v4l2-controls.h
>>> @@ -898,6 +898,8 @@ enum v4l2_mpeg_video_av1_level {
>>>       V4L2_MPEG_VIDEO_AV1_LEVEL_7_3 = 23
>>>   };
>>> +#define V4L2_CID_MPEG_VIDEO_AVERAGE_QP  (V4L2_CID_CODEC_BASE + 657)
>>> +
>>>   /*  MPEG-class control IDs specific to the CX2341x driver as defined by 
>>> V4L2 */
>>>   #define V4L2_CID_CODEC_CX2341X_BASE (V4L2_CTRL_CLASS_CODEC | 0x1000)
>>>   #define V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE 
>>> (V4L2_CID_CODEC_CX2341X_BASE+0)
>>